Prototheria
| Prototheria Temporal range: Late Triassic - Recent
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Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Prototheria Gill, 1872 |
| Groups included | |
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| Cladistically included but traditionally excluded taxa | |
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Prototheria (
Most of the animals in this group are extinct. The egg-laying monotremes are known from fossils of the Cretaceous and Cenozoic periods; they are represented today by the platypus and several species of echidna.
The names Prototheria,
The threefold division of living mammals into monotremes, marsupials and placentals was already well established when
Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis which defined the group by two supposed
- formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoidin therians; and
- a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range of Triconodonta and Multituberculata) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch.[4]
The evidence which was held to support this grouping is now universally discounted. In the first place, examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character which would support a 'prototherian' grouping of orders in contradistinction to Theria.[5]
In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps which is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the Australosphenida, and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians.[6][7]
The Australosphenida hypothesis remains controversial, and some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria.[8] In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order which can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.
Notes
- ^ "Prototheria". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2020-03-22.
- S2CID 247542433.
- ^ Marsupialia and Eutheria/Placentalia appear as cohorts in McKenna & Bell 1997 and in Benton 2005, with Theria ranked as a supercohort or an infralegion, respectively.
- OCLC 53970617.
- ISSN 0024-4082.
- S2CID 4342585.
- ISSN 0567-7920.
- OCLC 37345734.